1. Field of the Invention
This application relates generally to a navigation sensor, and more specifically to reducing silicon die size requirements for a navigation sensor through the use of an ultra high frame rate acquisition system.
2. Description of the Prior Art
A navigation sensor, such as may be used in a computer mouse, compares an incoming image to a reference image to determine the change in displacement in x and y directions. In order to track motion accurately, there must be sufficient overlap between the incoming image and the reference image because the overlap contains the signals required for the navigation to process. The required signals normally indicate microscopic features of the tracking surface, of which a comparison of respective locations in the incoming image and the reference image signify speed and direction of movement of the navigation sensor.
In a typical navigation sensor, image acquisition is frame based, meaning there is a fixed time interval between successive image acquisitions. For example, a 5,000 frame per second sensor will acquire an image every 200 us.
Take an example navigation sensor with a 20×20 array of pixels, each pixel being 30 um in size. Assuming an overlap between the incoming image and the reference image must be at least a 10×10 pixel block for good navigation processing, the navigation sensor has an effective maximum “speed limit” above which the overlap will be smaller than required, resulting in less than desired navigation information. FIG. 1 illustrates a 10×10 image overlap in such an example navigation sensor required for proper navigation processing by a digital signal processor (DSP).
With this example navigation sensor, the maximum speed limit can be calculated as a 10 pixel x-axis displacement through a 1:1 (no magnification) lens in a 200 us time interval is equal to a 300 um movement in 200 us. Therefore, the maximum speed limit in the x-axis is 300 um/200 us=1.5 m/s, or approximately 60 inches per second (ips).
In the event that the navigation sensor is moved above this maximum speed limit, the overlap between the incoming image and the reference image becomes too small. An example of where the overlap between the incoming image and the reference image becoming too small due to exceeding the maximum speed limit is shown in FIG. 2, which shows a possible only 5×5 overlap between the images, which is insufficient for proper navigation processing by the DSP.
As described above, a navigation sensor has an effective maximum speed limit for proper navigation processing. Because of the need to provide enough overlapping pixels between the incoming image and the reference image while still permitting enough motion between frames to be useful has placed a restriction on the minimum size of a conventional navigation sensor pixel array. Due to this restriction, the size of the navigation sensor array traditionally occupies approximately 30%-45% of expensive die area in the navigation sensor, a size range which is not easily shrinkable. Thus a navigation sensor has hitherto not benefited from the continuing technological trend of die miniaturization and associated reduction in costs.